1. Field of the Invention
This invention relates to a process and apparatus for drying moisture-containing particulate materials in a hot hydrocarbon liquid. More particularly, this invention provides a method and apparatus for drying wet ground coal in a hot hydrocarbon oil whereby the foaming of the coal-oil slurry is minimized.
2. Prior Art
In coal hydro liquefaction processes the moisture content of ground coal must be reduced to less than about 4 percent by weight before it is fed to the liquefaction reactor. In many state of the art liquefaction processes for example, ebullated bed processes, described in U.S. Pat. No. Re 25,770, it is necessary to slurry particulate coal with a liquid hydrocarbon prior to passage to the reaction zone. However, moisture contents greater than about 4 weight percent will usually produce some foaming in the slurry mixing tank and such foam will evolve at temperatures above about 220.degree. F. and foul the coal heating system as well as interfere with the smooth operation thereof. Therefore, it is desirable to remove as much of the moisture content in wet ground coal prior to passage to the liquefaction reactor.
The drying of high moisture-containing coal particulates by heating the particulates in a coal-oil slurry is well known. One such method is disclosed in U.S. Pat. No. 3,520,067 wherein wet ground coal is dried in a heated hydrocarbon oil maintained in a pressurized vessel. The water vapor liberated is withdrawn from the upper portion of the vessel as an overhead product. The moisture-containing particulates are introduced adjacent to, but below, the surface of the drying oil to minimize foaming and particle entrainment in the vapor products. This process does not provide any means for breaking the foam generated by the drying step.
Another prior art method for drying coal is disclosed in U.S. Pat. No. 3,680,217. According to this method, ground coal is dried prior to using in a coal hydrogenation liquefaction process and a heated slurry medium derived from the liquefaction process. The slurry containing the dried coal particulates is concentrated and used directly in the liquefaction process. However, since no means is provided for maintaining a well mixed slurry the coal will tend to settle out. In addition, this invention provides no means for controlling foaming.
A further prior art method is disclosed in U.S. Pat. No. 3,953,927, the disclosure of which is incorporated by reference, wherein coal particulates are dried by heating the coal in an oil slurry contained in a pressurized drying zone so as to vaporize the moisture. A principal portion of the heat utilized in the drying zone is provided by compressing the hot steam evolved from the coal drying zone and condensing it in the heat exchanger within the drying zone. This process suffers from the disadvantage that the evolution of moisture usually causes foaming and results in the formation of agglomerated coal particulates on the surface of the slurry which can clog the system unless sufficient mixing is provided. In addition, the coal particulates may be undesirably carried out of the vessel when they become entrained by the liberated water vapor.
The inventor has found that by means of the invention described herein, substantially all of the moisture content in wet ground coal may be removed without the disadvantages of the prior art.
It has also been discovered that by using the present invention, foaming in the hydrocarbon liquid is substantially reduced. Foaming is undesirable since it limits the rapid escape of water vapor from the hot slurry into the vapor stream. Furthermore, moisture may be entrapped in the foam so as to prevent complete drying of the coal particulates.